Paper products formed from lignocellulose pulps are commonly employed as insulation for various electrical apparatus. Such products are particularly useful for this purpose because of their dielectric strength and their economic advantage. The paper may be used, for example, as dielectric spacers in capacitors or as insulating sheet for windings in a transformer. Typically the electrical apparatus, encased in an appropriate housing, is immersed in a liquid dielectric such as petroleum oil, waxes or chlorinated hydrocarbons.
Paper insulating materials used in electrical apparatus are ordinarily subjected to elevated temperatures, and it has long been recognized that the insulating papers deteriorate rapidly in use. This deterioration still exists when the insulating papers are in contact with, or immersed in, a liquid dielectric such as transformer oils. The elevated temperatures may cause the liquid dielectric to break down into acids or other chemical constituents which attack or degrade the cellulose insulation material. As a consequence, the insulating paper gradually deteriorates thereby adversely affecting its electrical and mechanical properties. For this reason the paper is treated or impregnated with various materials or compounds to improve the electrical performance and stability of the paper.
A number of prior art patents disclose impregnating the paper with a nitrogen-donor compound or compounds in order to increase the nitrogen content of the paper thereby improving its insulating properties, especially thermal stability. Representative U.S. patents include U.S. Pat. No. 2,535,960 (impregnating the pulp with acrylonitrile); U.S. Pat. No. 3,102,159 (melamine and dicyandiamide added to the paper at the sizing tank); U.S. Pat. No. 3,469,219 (paper impregnated with a guanamine). Other prior art U.S. patents disclose sizing the paper with a protein such as casein or soybean protein. These patents include, for example, U.S. Pat. Nos. 2,339,707, 3,119,732, 3,166,466 and 3,328,184.
The prior art also discloses treating the paper simultaneously with a nitrogen donor compound and a protein. Representative U.S. patents disclosing this combination include U.S. Pat. Nos. 3,135,627, 3,211,516 and 3,224,902. Also U.S. Pat. No. 4,196,044 discloses a creping and calendering process for increasing the density of the paper by treating the paper with a creping compound (e.g., casein) and states generally that the paper product formed by the process may be sprayed or dipped with a nitrogen-donor compound (e.g., dicyandiamide).
The prior art teachings, however, are all deficient in one or more respects. There is no teaching or suggestion as to certain essential process conditions and/or process steps. We have found most unexpectedly that this combination of certain process conditions and steps is critical to the preparation of insulating paper exhibiting improved dielectric integrity. Equally significant, the prior art and the accepted practice in the electrical industry utilize exclusively high density papers (i.e., papers having a density greater than 0.9 gm/cm.sup.3, and typically 1.0 gm/cm.sup.3 or greater) and particularly in areas where electrical apparatus requires high dielectric strength. Low density paper inherently exhibits a low dielectric strength and therefore this is one factor that should prohibit its use as insulating paper. We have found, contrary to the prior art and industry practice, that our process is especially applicable to low density paper thereby improving its electrical integrity.
This invention has as its purpose to provide a process for preparing paper cellulose material, especially low density paper, exhibiting improved dielectric integrity, and the product formed therefrom. This together with other objects and advantages of the invention may be further understood by reference to the following detail description and accompanying drawings.